I think in a board with no (or light) trashing, IGG can give out curses significantly faster than witches, and is the better curser (solo games show all the curses to the opponent in 13-16 turns, this is almost a curse per turn, every turn, after turn 3!). The downside of course, is that your deck is now geared towards $5 a hand, rather than $8.
With decent trashing the witch becomes much better because, +2 cards is good even after the curses are gone.
The endgames of both these scenarios play out very differently, I think. After an IGG rush you pretty much have to rush a thrid pile (assuming both IGG and curses are gone), where with witches you probably still have 2-piles left to deplete and have to aim for provinces.